Video measuring machines are often required to measure test objects having features of different heights, including at different magnifications, which can require adjustments to capture images in proper focus. Although many such machines include telecentric imaging systems for avoiding variations in magnification through a depth of focus, adjustments are often required to maintain the test object or features of the test object at a best or at least acceptable focus position of the telecentric imaging systems for taking accurate measurements.
Focusing can be accomplished in a variety of ways including by relatively moving all or a portion of the imaging system with respect to the test object. Identifying suitable focusing positions can also be accomplished in a variety of ways, including by taking one of more measurements of image contrast within the captured images. For example, image contrast can be assessed by measures of intensity variations or energy content of higher spatial frequencies.
However, the surfaces of some test objects can be either poorly reflective or even too specular, which can diminish contrast variations within the captured images and thereby diminish the accuracy with which focus adjustments relying on contrast variations can be made. To overcome this type of obstacle, grid projectors have been added to video measuring machines for projecting a reticle grid onto the test objects. The projected grid, which can be readily imaged within the camera, provides the required contrast variation necessary for making proper focus adjustments.
Known video measuring machines can be arranged with a variety of illumination systems, including coaxial or through-the-lens illumination systems, oblique illumination systems, and backlighting illumination systems. Although LED (light emitting diode) and other efficient lighting sources can be used, a large amount of energy is sometimes required to achieve the proper level of illumination, which can heat the imaging system optics or their supports. Material expansions caused by heat, as well as adjustments between different settings, such as different magnifications, can affect optical system alignments and other machine calibration issues.
For purposes of monitoring and correcting such misalignments and related calibration issues, another type of grid projector has been used to project a grid through at least a portion of the imaging system onto the camera's image plane detector. The grid includes centering features, such as concentric circles, which can be used for monitoring image position and size on the camera detector.
The additional apparatus for projecting different grids in opposite directions through the imaging system adds cost, complexity and bulk to the imaging systems.